Integrated circuits are specified to operate within a desired power supply voltage range. However merely ensuring proper operation within the specified range alone may not be sufficient. Abrupt changes in circuit activity may cause the internal power supply voltage to temporarily overshoot or undershoot its specified range. If the integrated circuit were designed and tested to operate only within the specified range, then the temporary overshoot and undershoot would cause some of the integrated circuits to occasionally malfunction.
For example, a multi-core data processor can undergo large variations in circuit activity. Typical multi-core data processors have between four and thirty-two data processor cores, and a multi-threaded operating system can assign data processor cores different program threads. The operating system can cause the processor running the code in each thread to operate in its active state until it encounters a barrier. The barrier occurs when multiple threads are simultaneously waiting for a condition to occur or for an operand to become available. Upon encountering the barrier for a particular thread, the operating system causes the data processor core running that thread to enter an idle state. Later, when the operand becomes available or the condition occurs, then the operating system releases the data processor cores corresponding to the threads waiting at the barrier at about the same time. This sudden release causes a spike in activity and a large undershoot on the power supply voltage.
In order to compensate for occasional undershoot or overshoot, circuit designers frequently add a guardband to extend the voltage range within which the circuit will operate properly. However adding the guardband often requires designers to adopt more conservative design practices. For example, adding a large guardband in design may cause the integrated circuit to operate more slowly than would otherwise be possible to meet the extended power supply voltage range, and testing the integrated circuit with the large guardband may cause some otherwise functional integrated circuits to fail functional tests and to be discarded.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.